This invention relates generally to optical fibers, and specifically to an apparatus for generating a single sideband modulated optical carrier totally within an optical fiber.
The theory of single sideband (SSB) modulators is very well understood. They involve a carrier signal which has a frequency f.sub.c, and the amplitude or phase of which is modulated by a modulating signal having a frequency f.sub.m. The modulating signal typically carries the information which it is desired to transmit. Modulating the amplitude of the carrier signal with the modulating signal may be regarded as producing a new signal with a frequency spectrum having three discrete components one at the original carrier frequency f.sub.c and the other two occurring spectrally symmetrically to either side of f.sub.c at f.sub.c -f.sub.m and f.sub.c +f.sub.m. These symmetric side components are sidebands. Single sideband transmission is based on the observations that the carrier signal itself carries no information, and that the information carried in the two sidebands is essentially redundant. It is therefore possible to transmit the desired information by transmitting a single sideband, which is then decoded on the receiving end using the known carrier frequency. Producing a single sideband from a carrier signal and a modulating signal can be regarded as simply shifting the frequency of the latter by the former or vice versa, and so an apparatus for producing a single sideband signal is a frequency shifter.
There are numerous applications for single sideband modulators in communications and signal processing systems. Techniques for generating an SSB signal in radio and microwave systems are fairly well established.
It is known that a variation of these techniques can be adapted for modulation of an optical carrier. Sidebands can be generated by amplitude or phase modulation of the carrier. Filtering techniques such as those involving use of a Fabry-Perot interferometer can be used but such techniques involve bulky apparatus and sometimes require critical alignment. Furthermore, if the optical wave is travelling in a glass fiber, the light must be filtered outside of the fiber, requiring coupling lenses that result in coupling losses and, again, in the use of bulky components. Another approach using bulk optics is an acousto-optic Bragg cell wherein optical radiation is diffracted by an acoustic wave and simultaneously shifted in frequency by an amount equal to the acoustic frequency.
A known alternative to the use of bulk optics in obtaining SSB modulation of an optical carrier is the use of integrated optics (IO). One such technique involves use of an IO version of the Bragg cell, where the light is confined in a slab waveguide fabricated of a material such as Ti-diffused LiNbO.sub.3, and deflected by a surface acoustic wave. Another technique is based on a scheme first suggested by F. Heissman and R. Ulrich, "Integrated-Optical Single Sideband Modulator and Phase Shifter," IEEE JQE, Vol. QE-18, No. 4, April 1982 pp. 767-71. It involves spatially weighting the coupling between two nonsychronous waveguide modes at suitable positions along the two waveguides. This system has been reduced to practice on an IO chip with an electro-optical Bragg array of electrodes. In all known IO versions for SSB modulation, however, it is necessary to couple the light signal out of the optical fiber, into the waveguide, out of the waveguide, and back into the optical fiber.
In summary, while there exist known techniques for SSB modulation of a light signal, they involve either the use of bulk optics or coupling of a light signal to and from a waveguide. Until the present invention, there have been no known "in line" fiber optic devices which produce SSB modulation totally within the fiber.